In an image forming apparatus such as a multi function peripheral (MFP), as a device that fixes a toner on a sheet, a fixing device including a fixing roller that heats and melts the toner and a pressing roller that comes into press contact with the fixing roller to form a nip section between the pressing roller and the fixing roller and compression-bonds the melted toner on the sheet is used.
The fixing roller is heated by induction-heating a conductive layer of the fixing roller with, for example, an induction heating coil (IH coil) and heats and melts the toner with the heat of the induction heating. When the fixing roller is heated over the entire area in an axial direction thereof, if sheets having width smaller than a heating range of the fixing roller are continuously caused to pass, heat is not deprived by the sheets in areas on the outer sides where the sheets do not pass. Therefore, temperature rises higher than that in an area where the sheets pass.
To solve this problem of the temperature rise and realize energy saving, a fixing device in which an IH coil is divided in the width direction of a fixing roller is proposed. In such a fixing device, it is possible to feed an electric current only to IH coils arranged in a range corresponding to a range where sheets to be subjected to fixing processing pass and heat a necessary range of the fixing roller. Consequently, even when small-size sheets are continuously subjected to the fixing processing, since an area where the sheets do not pass is not heated, the temperature rise does not occur and the energy saving can be attained.
However, in general, the IH coil of the dividing type is divided into three blocks including an IH coil located in the center in a passing range of sheets and IH coils arranged on both the sides of the passing range. If the number of divisions is increased, a circuit for driving the IH coils is complicated and control of the IH coils is complicated to cause an increase in cost. Therefore, when the passing range of the continuously passing sheets and a range heated by the IH coils do not coincide with each other, in the fixing roller and a fixing belt, the temperature in non-sheet passing areas still rises to cause temperature fluctuation in the axial direction.